Background

Ocular melanoma is an uncommon malignancy, usually occurring in the uvea of the eye.

For asymptomatic patients and small tumors (<10 mm in diameter and <2 mm in height), observation is often an acceptable treatment.

For larger or symptomatic tumors, treatment with radiation is preferred over enucleation.

Radiation can be delivered using brachytherapy (radioactive plaques) or charged particle irradiation, or even stereotactic radiotherapy.

Enucleation is reserved for patients for whom radiation unlikely to provide local control, including large tumors, those with extrascleral extension, or painful eye.

Proton beam therapy is ideally suited to uveal melanomas as it is able to spare the remainder of the ocular structures. It also permits larger tumors to be treated than with plaque brachytherapy.

The largest experience of proton therapy for uveal melanomas comes from the Harvard Cyclotron Laboratory where over 2000 patients have been treated. Local control in these patients is 95% at 15 years and eye preservation in 84%. (Gragoudas Clin North Am 2010)

There is a need for more accurate and reliable techniques for eye tracking and beam gating.

The authors designed and evaluated a new real-time eye-tracking and beam-gating system for accurate proton-beam therapy of ocular tumors.

Methods

The authors developed a system by image-pattern matching, using a normalized cross-correlation technique with Labview 8.6 and Vision Assistant 8.6 (National Instruments, Austin, Texas, USA).

For consistency and safety, three different calibration modes in which the actual distance of an image pixels size can be measured in pixels were designed.

The three modes for calibration are point, edge, and line detection.

After calibration, a template image is acquired.

Lastly, the eye is imaged and the template image is compared to the treatment image. If there is appropriate matching, radiation treatment was delivered.

After these methods were applied, gating was performed, and dose distributions were evaluated.

Results

The errors in the three calibration modes were within +/-0.002 mm per pixel.

The accuracy of displacement measurement for a motion template showed an error of +/- 0.032 mm in the point-detection mode, +/- 0.011 in the edge-detection mode, and +/- 0.037 mm in the line-measurement mode.

The gating of both non-motion and motion phantoms and the gating of only non-motion phantoms showed errors of 0.83 and 4.95 mm in the lateral penumbra (80-20%), respectively, as well as differences in field size of 1.59 and 6.17 mm in dose coverage.

Radiation therapy was delivered in three patients, for whom eye movement was a combined average maximum of 0.85 mm.

Author's Conclusions

The eye-tracking and beam-gating system described here permits high-precision treatment using proton therapy instruments.

Unlike conventional eye-tracking systems, the protocol described here requires new techniques for motion detection and patient positioning in order to use calibration functions.

Scientific/Clinical Implications

The authors describe a novel eye-tracking and beam-gating system for uveal melanoma. Proton therapy is able to deliver a highly conformal dose distribution; the concern is that eye movements could move the tumor out of the field and therefore not deliver the appropriate dose. Therefore eye-tracking and beam gating is essential to successful treatment of uveal melanoma with protons.

This system yields quantitative real-time information about the movement of the eye with respect to a reference position (template image).

This study also highlights that eye movement is significant in these treatments, with a combined average maximum of 0.85 mm movement in the 3 patients treated according to this study.

One alternative to eye tracking and beam gating is eye fixation. This system will have to be compared to other systems in development including one previously described in the literature of an eye fixation aid attached to a noninvasive, relocatable Gill-Thomas-Cosman head frame used together with a simple eye-movement tracking system. (Jaywant J Appl Clin Med Phys 2003).